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Rotational barriers of disilane, hexafluorodisilane, and hexamethyldisilane: Ab initio , density functional, and molecular mechanics (MM3) studies
Author(s) -
Cho Soo Gyeong,
Rim One Kwon,
Park Gyoosoon
Publication year - 1997
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/(sici)1096-987x(199709)18:12<1523::aid-jcc9>3.0.co;2-l
Subject(s) - disilane , ab initio , chemistry , density functional theory , bond length , computational chemistry , molecular vibration , atomic physics , molecular physics , physics , molecule , silane , organic chemistry
We investigated structures, vibrational frequencies, and rotational barriers of disilane (Si 2 H 6 ), hexafluorodisilane (Si 2 F 6 ), and hexamethyldisilane (Si 2 Me 6 ) by using ab initio molecular orbital and density functional theories. We employed four different levels of theories (i.e., HF/6–31G*, MP2/6–31G*, BLYP/6–31G*, and B3LYP/6–31G*) to optimize the structures and to calculate the vibrational frequencies (except for Si 2 Me 6 at MP2/6–31G*). MP2/6–31G* calculations reproduce experimental bond lengths well, while BLYP/6–31G* calculations largely overestimate some bond lengths. Vibrational frequencies from density functional theories (BLYP/6–31G* and B3LYP/6–31G*) were in reasonably good agreement with experimental values without employing additional correction factors. We calculated the ΔG ‡ (298 K) values of the internal rotation by correcting zero‐point vibration energies, thermal vibration energies, and entropies. We performed CISD/6–31G*//MP2/6–31G* calculations and found the ΔG ‡ (298 K) values for the internal rotation of Si 2 H 6 , Si 2 F 6 , and Si 2 Me 6 to be 1.36, 2.06, and 2.69 kcal/mol, respectively. The performance of this level was verified by using G2 and G2(MP2) methods in Si 2 H 6 . According to our theoretical results, the ΔG ‡ (298 K) values were marginally greater than the ΔE ‡ (0 K) values in Si 2 F 6 and Si 2 Me 6 due to the contribution of the entropy. In Si 2 H 6 the ΔE ‡ (0 K) and ΔG ‡ (298 K) values were coincidently similar due to a cancellation of two opposing contributions between zero‐point and thermal vibrational energies, and entropies. Our calculated ΔG ‡ (298 K) values were in good agreement with experimental values published recently. In addition, we also performed MM3 calculations on Si 2 H 6 and Si 2 Me 6 . MM3 calculated rotational barriers and thermodynamic properties were compared with high level ab initio results. Based on this comparison, MM3 calculations reproduced high level ab initio results in rotational barriers and thermodynamic properties of Si 2 H 6 derivatives including vibrational energies and entropies, although large errors exist in some vibrational frequencies. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1523–1533, 1997